1. Field of the Invention
The invention relates generally to angular filters and, in particular, to an array of axially oriented resistive elements supported by dielectric material.
2. Description of the Prior Art
An angular filter is a device which passes or rejects an electromagnetic wave depending on the angle of incidence of this wave relative to the filter surface. Typically, angular filters are designed to pass a wave at normal incidence (broadside) and to provide rejection that increases with angle of incidence away from broadside. Such a filter offers the potential for reducing sidelobes in the radiation patterns of directive antennas. Several types of angular filters, also called spatial filters, have been invesitgated including multilayered dielectric filters (R. J. Mailloux, "Synthesis of Spatial Filters with Chebyshev Characteristics", IEEE Trans. Antennas and Progagation, pp. 174-181; March 1976), perforated metal-sheet filters (E. L. Rope, G. Tricoles, "An Angle Filter Containing Three Periodically Perforated Metallic Layers", IEEE AP-S Int. Symp. Digest, pp. 818-820; 1979) and multilayered metal-grid filters (R. J. Mailloux, "Studies of Metallic Grid Spatial Filers", IEEE Int. Symp. Digest, p. 551, 1977; P. R. Franchi, R. J. Mailloux, "Theoretical and Experimental Study of Metal Grid Angular Filters for Sidelobe Suppersion", IEEE Trans. Antennas and Propagation, pp. 445-450, May 1983; P. W. Hannan and J. F. Pedersen, "Investigation of Metal-Grid Angular Filters", Proc. 1980 Antenna Applications Symposium, Allerton Park, Ill., September 1980; and J. F. Pedersen, P. W. Hannan, "A Metal-Grid 5.times.5 Foot Angular Filter", IEEE AP-S Symp. Digest, pp. 471-474, 1982). Metal-grid angular filters are practical and can offer improved performance (reduced wide-angle sidelobes) when added to an antenna. However, such filters have certain limitations. One limitation of metal-grid filters is the inherent relation between the angular characteristic and the frequency characteristic of the filter. Typically this results in a useful frequency bandwidth that is not very wide. Another limitation, inherent in the resonant nature of such filters, is the need to construct them with tight dimensional tolerances. Failure to hold sufficiently tight tolerances can result in variations of transmission phase across the filter aperture for incidence angles within the filter angular passband. Such phase variations can create unwanted sidelobes in the pattern of the antenna/filter combination. A third limitation of such filters is that they reject power by reflection rather than by absorption. This reflected power can return to the antenna that is associated with the filter, and then may reflect again to yield unwanted sidelobes within the angular passband of the filter.